Agree at the same time to withhold rent (6)
I believe the answer is:
concur
'agree' is the definition.
(synonyms)
'at the same time to withhold rent' is the wordplay.
I cannot quite see how this works, but
'at' could be 'on' (location) and 'on' is located in the answer.
This explanation may well be incorrect...
(Other definitions for concur that I've seen before include "Agree, assent" , "Agree or happen together" , "meet" , "run together" , "Express agreement" .)